Chairs falling within this category include a front pair and rear pair of crossed, pivotally interconnected legs, secured together by lower reinforcing cross-bars at each side and upper reinforcing seat-supporting rails at each side, the associated combined armrest and backrest structure being pivotally mounted upon the leg structure at each side and above the seat-supporting rails, the whole being adapted for swinging movement in unison from a collapsed position in side-by-side relation to a fully erect position in spaced-apart relation and reversely.
The seat element and backrest element are flexible and are adapted to extend respectively between the side or seat rails and the backrest structure, the seat and backrest elements assuming a somewhat taut condition with the seat rails and backrest structure in the spaced apart fully erect configuration, the legs preferably being provided with interlocking braces to releasably secure the legs against collapse when in chair defining relation and so stabilize the entire framework.
One example of an early chair structure of interest is that found in U.S. Pat. No. 1,739,552 wherein the upper side or seat rails are integral with the respective armrest and backrest structures and together are pivotally interconnected with the leg structure by offset or arm-like hinge members, the flexible seat element being secured by clamping strips thereto and stretched to tautness between the seat rails in chair defining relation.
Another prior alternative is the folding chair revealed by U.S. Pat. No. 2,962,085 in which metal side or seat rails have a specially extruded shape in cross-section and are fixedly secured to the opposed pairs of legs uppermost, each side or seat rail being hinged outermost throughout its entire extent to a like cooperating, rigidifying bar presented by the respective asociated armrest and backrest structure so as to securely clamp the flexible seat element against dislodgment when stretched therebetween in chair defining relation.
Still another embodiment of a collapsible chair demonstrating the trend of development in this field is that disclosed by U.S. Pat. No. 3,138,401 in which a combination metal hinge and leg bracket for each leg is provided for attaching and retaining the flexible fabric at the corners of the seat element which eliminates the need for a seat rail of substantial rigidity and strength thereby achieving a certain economy in production.